A state of the art bacterial-based wastewater treatment system and method 100 is shown in FIG. 1. The system and method 100 includes providing a waste source 102 such as a municipal sewer line. A wastewater stream 104 from the waste source 102 has an undesirable biological oxygen demand (BOD), and high nitrogen and phosphorus concentrations. This wastewater stream may also have other undesirable chemical compounds broadly classified as metals, neurotoxins, or endocrine disrupting compounds.
The wastewater stream 104 from the waste source 102 typically undergoes a primary treatment 106 where the wastewater stream 104 flows through large tanks, commonly called “primary clarifiers” or “primary sedimentation tanks”. The primary treatment 106 may be preceded by a pre-treatment (not shown) where materials that can be easily collected from wastewater stream 104 are removed, for example, by screening before they reach the primary clarifiers. The purpose of the primary sedimentation stage is to produce both a generally homogeneous influent 108 capable of being treated biologically, and a primary clarifier sludge (not shown) that can be separately treated and processed. The primary clarifiers usually are equipped with mechanically driven scrapers that continually drive the collected sludge 110 towards a hopper in the base of the tank, from which the collected sludge 110 can be pumped to further sludge treatment stages. Grease and oil from the floating material can sometimes be recovered for saponification during the primary treatment 106.
The effluent 108 from the primary treatment 106 then becomes the influent 108 to secondary treatment step 112 that is designed to substantially degrade the biological content of the influent 108. In a particular embodiment, the influent 108 is treated by an activated sludge process that uses aerobic bacteria to biologically remove organic matter from the influent 108. Under aerobic conditions provided by an input of oxygen, typically by bubble aeration, the bacteria consume the organic matter while generating new biomass and carbon dioxide. During the activated sludge process, bacteria growth tends to form large aggregates or flocs. In a particular embodiment, these flocs can settle to the bottom of additional secondary clarifiers, where the flocs of bacteria become the activated sludge. This sludge can be pumped to further sludge treatment stages or pumped back into the aerobic treatment stage to aid in consuming more organic matter.
The influent 108 can then undergo a tertiary treatment 116 which is intended to remove some portion of the remaining nitrogen and phosphorus. A carbon source 118 such as a methanol stream 120 may be added during the tertiary treatment 116 to aid in the process of de-nitrification where ammonia is converted to nitrites, and the nitrates are ultimately converted to nitrogen gas. In a particular embodiment, chemical inputs such as aluminum sulfate or iron salts are added to bind with phosphorus and form flocs that settle out in gravity clarifiers.
Following the tertiary treatment 116 of the influent 108, a disinfection treatment 122 may be conducted to produce a treated effluent 124 for release to the environment 126 such as a river. The purpose of the disinfection treatment 122 is to substantially reduce the quantity of microorganisms such as the aerobic bacteria to be discharged back into the environment 126. The effectiveness of disinfection depends on the quality of the influent 108 being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables.
Nutrient removal in conventional wastewater treatment processes such as the bacterial-based wastewater treatment system and method 100 described above is known to be inefficient. Removal of organic solids from the wastewater stream 104, for example, with chemical flocculants and centrifuges, is particularly problematic.
It is also known to use aquatic microorganisms such as algae in wastewater treatment. Particular systems and methods employing algae in wastewater treatment are described in U.S. Pat. No. 6,465,240 to Wexler et al. and U.S. Pat. No. 6,896,804 to Haerther et al., the entire disclosures of which are hereby incorporated herein by reference. Wexler et al. discloses a method for treating a waste stream by contacting the waste stream sequentially with a consortium of prokaryotic microorganisms, preferably purple non-sulfur bacteria, followed by a green algae, Chlorella. The consortium of prokaryotic microorganisms assimilates a first portion of the wastes, and the green algae assimilate the remaining portion of the wastes to produce a substantially purified effluent stream. Haerther et al. discloses a system and method provided for aerobic treatment of waste, which includes the continual introduction of microalgae. The high amounts of oxygen produced by the microalgae satisfy the BOD in the treatment process and also allow oxidation of undesirable contaminants. These known systems and methods using algae undesirably require photosynthesis in order to grow the algae and treat the wastewater.
Another algal-based system and method for wastewater treatment is described by Nakajima et al. in A Photo-Bioreactor using Algal Phototaxis for Solids-Liquid Separation, Wat. Res. Vol. 25, No. 10, pp. 1243-1247, 1991, the entire disclosure of which is hereby incorporated herein by reference. Nakajima et al. uses the positive phototaxis characteristics of Euglena gracilis to separate the algal biomass from wastewater, following removal of nutritive substances from the wastewater by the Euglena gracilis. 
Generating renewable biomass in wastewater using such methods is difficult. Specifically, most algae require abundant natural light in order to drive photosynthesis and grow the biomass. It is also difficult to limit an algae population to a single species of algae, and prevent other microorganisms from competing in the holding tanks or ponds. Separation of the biomass from an aqueous environment in which the biomass is grown is also difficult due to the generally small cell size of the aquatic organisms used for the wastewater treatment and relatively dilute concentrations of algae biomass (e.g. typically far less than 1% solids).
There is a continuing need for a system and method for treating high-strength wastewater having high BOO, and high nitrogen and phosphorus concentrations, which produces a valuable biomass by-product that can be used as a feedstock for ethanol production or other bioenergy production. There is a further need for a system and method that also optimizes microorganism biomass harvesting. Desirably, the system and method can be used as a pre-treatment for industrial wastewater producers or in municipal wastewater treatment plants to permanently remove BOD, nitrogen, and phosphorus from the wastewater stream.